The invention relates to a method for optimising coverage, useful in a TDMA-based radio system which comprises several terminal equipments and base stations, and in which a signal is sent in time slots, a propagation delay of the signal between the terminal equipment and the base station is defined, and the antenna lobe of the base station is oriented.
An example of a typical time divisional radio system is the GSM (Global System for Mobile communication). Multiple access is implemented by providing several carrier waves, each carrier wave having at most eight simultaneous users. In order that the signals of these eight users could be separated from one another, they are sent as bursts in predefined time slots. In other words, a transmitter transmits only at a predefined time instant. A frame comprises the time slots of eight users, and repetition of the frame allows transmission of even a large quantity of data.
In prior art solutions, the coverage of a multiple access cellular radio system based on time division is optimised by adjusting the transmission power and/or orienting the antenna lobe in the horizontal direction. The adjustment of the transmission power, however, is not always possible. In particular, when the receiver is located far or behind an obstacle, the transmitter has to use a high transmission power. The problem with horizontal orientation of the antenna lobe is that the base station has to know in what direction or location the terminal equipment is. This piece of information is not always available. Typically the base station knows only the propagation delay of the signal between the base station and the terminal equipment.
The object of the present invention is to optimise the coverage area of a base station in a situation where the base station does not know the location or direction of a terminal equipment. If, on the other hand, the base station knows the location or direction of the terminal equipment, the coverage area and the shape of the lobe can be further optimised more effectively than in the prior art.
The object is achieved by a method as described in the introduction, the method being characterized in that the antenna lobe of the base station assigned to the terminal equipment is oriented subscriber-terminal-specifically by changing the shape of the antenna lobe at least in the vertical direction.
Major advantages are achieved by the method of the invention. The orientation of the antenna lobe of the base station reduces the amount of transmission carried outside the cell, and so intercellular interference is also reduced. When the antenna lobe is oriented toward a terminal equipment, the transmission power of the terminal equipment can also be minimised, which reduces the overall interference level both within a cell and in adjacent cells.